We have to use a website from a vendor whose web application is only compliant with IE7. IE7 isn't even safe to use today, is it? I wouldn't recommend IE at all, but if it's necessary I wouldn't think anything older than IE8 would be even remotely safe. But this is really more of an assumption on my part based on the fact that the current version is IE10 and IE11 is on the horizon.
According to the life cycle info I could find on IE (Wikipedia (I know, I know)), it appears the IE version that ships with a given Windows OS is supported by Microsoft for the life of the OS. Is that really the case? Do they really keep all of these IE versions updated security-wise INCLUDING 7?